In surgical implants or instruments, it is sometimes necessary to secure two mutually displaceably guided parts against displacement with a screw. It is known to do this by screwing a fixing screw through a passage orifice in the upper of the two parts and into a threaded bore in the lower of the two parts until its head sits firmly on the top face of the upper of the two parts. This has the disadvantage that the screw can be lost, which can have serious consequences in a confined operating site.
It is known (EP-A-589325, EP-A-1031332) to avoid this problem by using a screw which is designed as a set screw and can be accommodated over its entire length in the threaded bore and in any case beneath the lower face of the upper of the two parts. Such a screw has an upper, thinner dog point which contains a groove for a screwdriver. The diameter of the passage orifice is smaller than that of the screw thread but larger than that of the screw dog point. Before the parts are joined together, the screw is screwed into the threaded bore to such a depth that it disappears completely in the threaded bore or at any rate lies deep enough within the bore that the upper of the two parts can slide over it. The screwdriver is then engaged through the passage orifice of the upper part and is used to turn the screw upward until its dog point lies in the passage orifice. The two parts are now secured against relative displacement so long as the screw is turned tight. If the screw loosens, however, it may gradually drift downward to such an extent that the fixing action is compromised.